The embodiments described herein relate to generating and maintaining the conductivity of microfractures in tight formation by generating gas and heat.
Subterranean wells (e.g., hydrocarbon producing wells, gas producing wells, water producing wells, and the like) are often stimulated by hydraulic fracturing treatments. In traditional hydraulic fracturing treatments, a treatment fluid, which may also function simultaneously or subsequently as a carrier fluid, is pumped into a portion of a subterranean formation (which may also be referred to herein simply as a “formation”) at a rate and pressure sufficient to break down the formation and create one or more fractures therein. Typically, particulate solids, such as graded sand, are suspended in a portion of the treatment fluid and then deposited into the fractures. The particulate solids, known as “proppant particulates” (which may also be referred to herein as “proppant” or “propping particulates”) serve to prevent the fractures from fully closing once the hydraulic pressure is removed. By keeping the fractures from fully closing, the proppant particulates aid in forming conductive paths through which fluids produced from the formation flow, referred to as a “proppant pack.” The degree of success of a stimulation operation depends, at least in part, upon the porosity of the proppant pack to permit the flow of fluids through the interconnected interstitial spaces between abutting proppant particulates.
In the case of stimulating low permeability formations (or “low permeability formations”), such as shale reservoirs or tight-gas sands, increasing fracture complexity during stimulation may enhance the production of the formation. Low permeability formations, such as those described herein, tend to have a naturally occurring network of multiple interconnected fractures referred to as “fracture complexity.” As used herein, the term “low permeability formation” refers to a formation that has a matrix permeability of less than 1,000 microDarcy (equivalent to 1 milliDarcy). As used herein, the term “ultra-low permeability formation” refers to a formation that has a matrix permeability of less than 1 microDarcy (equivalent to 0.001 milliDarcy). As used herein, unless otherwise stated, the term “fracture” or “fractures” will refer collectively to micro-sized fractures (referred to herein as “microfractures”) and fractures having larger openings (referred to herein as “macrofractures”).
Fracture complexity may be enhanced by stimulation (e.g., fracturing) operations to create new or enhance (e.g., elongate or widen) existing fractures. In such cases, the newly formed fractures may remain open without the assistance of proppant or micro-proppant particulates due to shear offset of the formation forming the fractures, or may have included therein proppant or micro-proppant particulates, depending on the size of the fracture, to assist in keeping them open after hydraulic pressure is removed. The inclusion of proppant or micro-proppant particulates in the fractures, new or natural, may increase the conductivity of a low permeability formation.
In some cases, subterranean treatment operations (e.g., stimulation, proppant placement, micro-proppant placement, and the like), may be supplemented with enhanced oil recovery techniques. Such enhanced oil recovery techniques may operate to enhance the conductivity of fractures. One such technique is acidizing, which involves injecting an acid (e.g., hydrochloric acid) into a subterranean formation in order to etch channels or create microfractures in the face of the formation and/or within an existing macrofracture or microfracture, thereby enhancing the conductivity of the formation. The acid may create a branched, dendritic-like network of channels through which produced fluids may flow.
Acidizing may operate to supplement or enhance the conductivity and production of the formation. Acidizing treatments are preferentially performed at multiple intervals or zones in a subterranean formation so as to maximize fracture complexity. However, such multiple interval treatments may be limited due to acid spending or leak off. If the acid is spent or experiences leak off prior to reaching one or more desired intervals of the subterranean formation (e.g., by leak off in a first or earlier contacted desired interval), it may be insufficiently potent to etch channels and, thus, may not contribute to or may only minimally contribute to enhancing fracture complexity and conductivity.